The present invention refers to a positioning system, in particular for acquiring Georadar type data, and more particularly of the 3-dimension type.
The laying of new technological networks (optical fibers) and the new lines for conveying electricity is based on no-dig or trenchless technologies. This technology is characterized by the possibility of laying the plants without having to dig a trench. A remote-control drill drags the cable along a preset path. The intensive laying of cables and tubes in the first meters underground and the lack of maps of the technological plants quite often make surveys necessary prior to the laying phases.
The Georadar surveys are up to now the only ones capable of identifying any technological plant or construction present underground.
During the laying of a cable without digging a trench, the more dangerous portions of the trench are the drill immersion and surfacing areas, that have a plan extension of about 4 m*4 m for a depth of a few meters, where the majority of the subservices can be found. The drill is immersed to the preset depth and then advances horizontally parallel to the plane (surface) of the ground.
In addition, this laying technique is suggested for laying the new technological networks (Directive dated 3 Mar. 1999 art.5).
The geophysical radar or Georadar uses the reflection of electromagnetic waves for exploring underground. The technique is based on the insertion in the ground, by means of a transmitting antenna, of electromagnetic waves with extremely brief impulses (a few nanoseconds), whose central frequency is between 10 and 2000 MHz. The signal reflected from the surfaces of any discontinuity present underground is received by a receiving antenna and recorded after sampling at suitable frequency. This type of survey enables high resolution measurements to be carried out with graphic return of an underground continual section (time-distance) in real time
To analyse an area of ground, the transmission and reception antennas have to be passed over the entire area concerned. The depth to be surveyed and the resolution required are determined through the choice of the antenna. High frequency antennas have good resolution but low penetration. To conduct Georadar surveys to locate the subservices the resolution needed is a few centimeters and the survey depth required is a few meters. Antennas with a frequency of the central band around 400 MHz are generally used.
The processing of the data received can be made by means of a bidimensional analysis, supplying in output the data relating to the various vertical planes analyzed. It can also be made by means of a three-dimension analysis. For three-dimension analyses the data received have to meet the theorem of the spatial sampling. In addition, good contact has to be kept and therefore good coupling between the ground and the antenna, as if there is air between the antenna and the ground the quality of the data deteriorates because of the reduced penetration of the signals.
Normally, for acquiring data, the lines that will indicate the path that the antennas will have to follow are traced on the portion of ground to be analyzed, by means of paint or chalk, or theodolites are used.
In addition, the majority of times, the Georadar surveys have to be carried out in heavily urbanized areas where it is very difficult to interrupt the traffic of vehicles, pedestrians or commercial activities. Therefore, they are operations that have to be carried out quickly minimizing as much as possible the disturbance to the road traffic and the surface activities.